Thermal spraying, also known as flame spraying, involves the heat-softening of heat-fusible material, such as a metal or ceramic, and the propelling of the softened material in particulate form against a surface to be coated to which the heat-fusible material bonds. A thermal spray gun is usually used for this purpose and, with one type, the heat-fusible material is supplied in powder form to the gun. The powder is of quite small particle size, e.g., below about 100 mesh U.S. Standard screen size to as small as one micron, and is difficult to meter and control.
A thermal spray gun normally utilizes a combustion or plasma flame to effect melting of the powder, but other heating means, such as electric arcs, resistance heaters or induction heaters can also be used, alone or in combination. In a powder-type combustion thermal spray gun, the carrier gas for the powder can be one of the combustion gases or compressed air. In a plasma spray gun, the carrier gas is generally the same as the primary plasma gas, although other gases such as hydrocarbon are used in special cases.
To obtain high quality coatings, it is necessary to accurately control the rate of the powder fed through the gun and to maintain the rate constant for a given set of spray conditions. The type of fine powder used is a very difficult material to handle and to feed with any uniformity into a carrier gas. While various apparatus of different designs and modes of operation based on gravity, mechanical and gas conveying, and combinations thereof, have been proposed such devices almost universally suffer from a lack of reliability in maintaining a constant controlled powder feed rate and are often subject to mechanical wear and breakdown. A contributing factor is the wide range of powder sizes, materials and particle shapes used for thermal spraying.
The present invention pertains to and is an improvement over the thermal spray powder feeder of the general types described in U.S. Pat. Nos. 3,976,332 and 4,381,898. In U.S. Pat. No. 3,976,332, for example, there is disclosed a powder feeding system comprising an enclosed hopper for containing powder in loose particulate form. A carrier gas conduit connected to a carrier gas supply extends through the hopper in its lower portion and continues to a point of powder-carrier gas utilization. The carrier gas conduit has connected thereto a powder intake orifice which extends into the hopper below the level of the powder and has a geometric design and arrangement such that there is no gravity flow of the powder therethrough into a carrier gas stream in the carrier gas conduit in the absence of a fluidizing gas flow therethrough.
Fluidizing gas in a regulated amount is supplied to the hopper, for example, above the level of solids therein so that in passing to the orifice the gas must pass through the mass of solids and be diffused thereby. The design of the hopper is such that the gas converges towards the powder intake conduit and fluidizes the powder in a fluidized zone in the immediate vicinity thereof, the powder surrounding the fluidized zone being non-fluidized and acting as a diffusion region for introducing the fluidized gas uniformly into the fluidized zone.
As further disclosed in U.S. Pat. No. 3,976,332, the carrier gas is supplied in a predetermined, constant amount. The flow of the fluidizing gas is regulated in a manner disclosed in U.S. Pat. No. 3,501,097, by sensing the pressure at a point in the carrier gas line, which pressure is responsive to the mass flow rate of solids therethrough, and then using the change in the pressure in the conveying gas line, if any, to regulate the flow of the fluidizing gas. If the pressure should increase, the flow of the fluidizing gas is made to decrease, and vice versa.
It has been found that the type of system of U.S. Pat. No. 3,976,332 has excellent repeatability and uniform control of the powder feed rate. However, certain problems became apparent, especially with very fine, difficult-to-feed ceramic powders. One such problem is pulsation, apparently due to a pressure oscillation, resulting in uneven thermal sprayed coating layers. Experimental use of several powder intake conduits relieved this problem but another problem developed, which was a continuation of powder feeding when the fluidizing gas is shut off. This continuation of feeding has been speculated to be due to a portion of carrier gas exiting one intake conduit and carrying powder into another.
Therefore, an object of the present invention is to provide an improved powder feeding system for a thermal spray gun which provides uniform control of powder feed rate with reduced pulsation and which does not feed into the carrier gas during idle mode.
Another object is to provide a novel powder pickup device for a powder feeding system which provides improved control of the powder feeding.